Recently, in this country there has been a tendency to reduce dependency on petroleum in view of the insufficiency of the supply of crude oil, by changing crude oil burning industrial plants to coal burning plants. In particular, with industrial thermal power generation boilers, large capacity thermal power plants especially for coal are now under construction.
On the other hand, it is a feature of recent electric power demand that a difference in load between a maximum load and a minimum load is increased with the increase of the atomic power generation. There is a tendency that boilers for thermal power generation are modified from those for base loads to load adjustment type boilers. It is possible to enhance the power generation efficiency by several percent under a partial load operation, by a variable pressure operation boiler which operates in a super critical pressure region in a so-called full load operation mode in which the thermal power generation boilers are operated by changing pressures in response to the load and which operates in a subcritical pressure region in a partial load operation mode.
For that reason, there is a decreased number of boilers especially for burning coal, which are always operated under the full load. The boilers designed especially for burning coal are converted to boilers for intermediate loads, which perform a so-called daily start/stop (hereinafter simply referred to as DSS) operational mode in which in the daytime the load is changed among 75% load, 50% load and 25% load and in the night-time the operation is stopped.
Also, there are a small number of the coal burning boilers for performing the DSS operational mode, which operate over the full load region only with pulverized coal from the start to the full load operation. Even in the coal burning boilers, readily ignitable auxiliary fuel such as light oil, heavy oil, gas and etc. other than pulverized coal is used for the start or low-load operation.
The reason for this is that, in the starting mode, it is impossible to obtain exhaust gas or heated air from the boiler for warming up a mill, and hence it is impossible to operate the mill for pulverizing coal.
Also, the light oil, heavy oil and gas are used for the reasons that it is impossible to keep a "turn down" ratio of the mill under the low load, and the ignition property of the pulverized coal itself is deteriorated.
For example, in the case where the light oil and heavy oil are used in the starting mode, the light oil is burned as fuel from the start to the 15% load, the fuel is switched over from the light oil to the heavy oil in the range of 15% load to 40% load, and over 40% load the mixture of the heavy oil and the pulverized coal is burned and over 40% load, the mixture is burned while gradually decreasing the amount of the heavy oil and gradually increasing the amount of the pulverized coal, thereby increasing the mixture burning rate of the pulverized coal for transferring to substantial coal burning.
On the other hand, in the case where the boiler load is reduced from full load to the low load, the pulverized coal fuel is burned to 35% load to operate the boiler as the coal burning boiler, and below the 35% load, the boiler is operated by auxiliary fuel such as heavy oil, light oil and gas.
As described above, in the coal burning thermal power plant which performs the DSS operational mode, it is normal to use the pulverized coal and the readily ignitable auxiliary fuel such as light oil, heavy oil and gas.
FIG. 4 is a schematic diagram showing a conventional pulverized coal burning boiler.
In FIG. 4, pulverized coal burners 4, 5, 6, 7, 8 and 9 are arranged from a bottom to a top of a boiler furnace 1 in a front wall 2 and a rear wall 3 of the boiler furnace 1.
After-air-ports 10 and 11 are provided above the pulverized coal burners 8 and 9 for reducing generation of NOx. Air is supplied from front wall wind boxes 12 and rear wall wind boxes 13 to the respective pulverized coal burners 4, 5, 6, 7, 8 and 9 and from a front wall wind box 14 and a rear wall wind box 15 to the after-air-ports 10 and 11, respectively.
The supply of coal to the pulverized coal burners 4, 5, 6, 7, 8 and 9 is performed as follows. Coal within a coal banker 16 is fed from a coal feeder 17 to a mill 18 and is pulverized within the mill 18. Rough particles contained in the pulverized coal in the mill 18 are classified by a classifier (not shown) and are returned back to the pulverizing section within the mill 18. The pulverized coal is fed from the pulverized coal supplying means, i.e., the mill 18 to the respective burners 4, 5, 6, 7, 8 and 9 by the delivery means including pulverized coal pipes 23, a blower 50 and the like. The delivery of the pulverized coal is performed by generating air flow from an air duct 22 through the mill 18 and the pulverized coal pipes 23 to the burners 4, 5, 6, 7, 8 and 9 by means of the blower 50.
The burning air for the front wall wind boxes 12, the rear wall wind boxes 13, the front wall wind box 14 and the rear wall wind box 15 is pressurized by a forced draft fan 19 and thereafter is preheated in an air heater 20. The air is supplied through an air passage 21, an air flow adjustment damper or restrictor 24 and air passage 25 to the wind boxes 12, 13, 14 and 15.
Gas is supplied to a hopper 26 by a gas recirculation fan 27 and a gas recirculation passage 28 for controlling a steam temperature during a partial load of the boiler. A gas duct 29 for mixing the gas with the burning air of an air passage 25 from an outlet of the gas recirculation fan 27 is provided for reducing the generation of the NOx.
The foregoing description relates to the general flows of the burning air, the gas and the pulverized coal in the pulverized coal boiler. It should be noted that the respective burners 4, 5, 6, 7, 8 and 9 are provided with igniters.
FIG. 5 is a enlarged view showing a detail of the fine coal burner shown in FIG. 4.
In FIG. 5, the reference numerals are used to show the same components as in FIG. 4. Reference numeral 1 denotes a boiler furnace, numeral 2 denotes a front wall, numeral 3 denotes a rear wall, numerals 4, 5, 6, 7, 8 and 9 denote pulverized coal burners, numerals 12 and 13 denote front and rear wall wind boxed, respectively, and reference numeral 23 denotes a pulverized coal pipe.
Reference numeral 30 denotes an air resistor and reference numeral 31 denotes a plasma igniter.
The development and research of a pilot burner wherein auxiliary fuel having good ignitability such as light oil, heavy oil, gas and the like for a pulverized coal boiler have been advanced. As a typical example, the development of the device provided with a plasma igniter 31 for directly igniting the pulverized coal using a plasma arc is realized as shown in FIG. 5. The igniter device provided with such a plasma igniter 31 is of the type such that a high temperature heat source at 1,500.degree. to 2,000.degree. C. is provided for directly igniting the pulverized coal without any auxiliary fuel such as light oil, heavy oil and gas. However, this system involves a serious problem and is not actually used since the ignition of the plasma arc needs a heat source near to 2,000.degree. C. with large energy of 60 to 80 kW at the ignition stage, thereby discharging a great amount of thermal NOx at ignition with the pulverized coal burners 4, 5, 6, 7, 8 and 9.
In the conventional pulverized coal burning boiler, the light oil and heavy oil that have a good ignitability are used as auxiliary fuel, and in the load change mode due to the DSS operation, the heavy oil is used for the burner starting fuel and the light oil is used for the igniters in view of the ignitability and operability. Three different kinds of fuel including pulverized coal as the main fuel are needed for the conventional boiler. Thus, there is a disadvantage that the instrument cost and running cost in connection with the transportation, storage and maintenance of the respective fuels are required.
Also, in the direct ignition system using the plasma arc as described above, the ignition energy and the thermal source temperature are too high. Therefore, this system causes such a problem that a great amount of NOx is generated in the ignition operation.
In view of the foregoing defects inherent to the conventional system, an object of the invention is to provide a burner device for directly igniting the pulverized coal with high ignition reliability without discharging NOx larger than necessary and without auxiliary fuel.
Incidentally, Japanese Patent Unexamined Publication No. 61-184309 and U.S. Pat. No. 4,545,307 show the other prior art relating to the present invention.